Can't view uploaded video's

I hope this is the correct forum to ask this questions.

I'm having trouble uploaded and playing video's since upgrading to wp 2.9.2.
Video's that were already posted on the site before upgrading, now appear as dead links.

I repost them and a day or two later they are dead again.

When I upload now and try to playback I get only a "share button" option from the player.
Does anyone have any ideas that are simple! I'm not a computer geek. So plan language please.